Subaru EJ25 spark plug tube gasket

These SUBARU spark plug tube gaskets shrink with age and can allow engine oil to seep into the spark plug tubes. If enough engine oil builds up, cylinder misfire such as P0301 P0302 P0303 P0304 can occur.


With the heat from the engine oil contacting these seals, and since they are under constant compression under the valve cover, eventually they get stiff and can start to let oil leak into the spark plug tubes.

Replacement of these seals on the Subaru 2.5L engine requires removing the valve covers. Valve cover removal is fairly simple, requiring a 10mm ratchet wrench or socket. Replacing the valve cover gaskets is a good idea if they are old or weeping. No gasket sealant or RTV is necessary.

If the oil soaks the spark plug wires where they sit in the spark, they can become cracked or otherwise damaged. The engine oil is conductive enough at the voltage and frequency of the spark plug signal to conduct the spark away, which will cause misfire in that cylinder.